Surface energy is determined by the chemical composition of the surface and is the result of intermolecular (or in the case of atomic substances, interatomic) attraction. These attractions may be non-specific intermolecular attractions due to the van der Waals-type interactions that exist between all atoms and molecules. These interactions are due to random and transitory fluctuations in electron cloud density that create temporary dipoles which attract each other. Some attractions may also be specific intermolecular attractions between permanent dipoles, between permanent dipoles and induced dipoles, or they may result from electron transfer type interactions such as Lewis type acid-base interactions. Because there are several types of intermolecular interactions responsible for surface tension, there are several components to surface tension. Therefore, surface energy is a multidimensional parameter with terms representing the contributions from each type of intermolecular interaction. To completely determine surface energy requires obtaining as many measurements as there are parameters, typically two or three at a minimum.
Measuring the contact angle of a series of liquid drops on a surface of interest is one frequently used technique to determine surface energy. The contact angle θ that a liquid forms with a surface is determined by three parameters: the surface energy of the surface (γs), the surface energy of the liquid (γl), and the interfacial energy between the liquid and the surface (γsl), and is described by the Young Equation:
                    θ        =                              cos                          -              1                                (                                                    γ                s                            -                              γ                sl                                                    γ              l                                )                                    (        1        )            
Contact angles are usually measured using a device known as a contact angle goniometer. A drop of a probe liquid is placed on the surface to be interrogated and allowed to spread equilibrium. The plane of the surface with the drop of liquid is brought into the line of sight of a microscope containing a measuring scale, and a reticle in the microscope is made tangent to the drop profile at the point of contact with the surface. The angle that the reticle makes with the surface is defined as the contact angle. Additional methods have been developed to calculate the contact angle based on the drop shape. These methods depend on obtaining an image of the drop profile.
Contact angle measurements are typically performed in a laboratory setting using cumbersome and delicate devices to take the measurements on small sections of material. These measurements are generally difficult to perform on large planar surfaces found in shop floor settings, particularly ones that are parts of large structures such as automobiles and aircraft. These measurements are also difficult to perform on surfaces that are curved, inclined, or inverted. Unfortunately, many surfaces of interest for contact angle measurements are parts of large, complex structures; and many of these surfaces are also curved, inclined, or inverted. This makes obtaining contact angle measurements on these surfaces very difficult or impossible using current techniques. Also, surfaces that are chemically or physically heterogeneous present additional obstacles to accurately measuring contact angles. The heterogeneities in a surface of a material tend to pin the drop perimeter during spreading, thereby acting as barriers that tend to prevent the drops of liquid from spreading to their equilibrium shape and establishing an equilibrium contact angle. This can introduce significant error into the measurements and result in erroneous values for surface energy.
A need exists for a device and method that can quickly and quantitatively probe the wetting characteristics of the surface of a material or an object. Another need is for a robust device and method that can allow these measurements to be conveniently obtained by semiskilled or unskilled workers in a manufacturing environment. Another need is for a device and method that can perform these measurements on surfaces that are non-planar. Another need is for a device and method that can perform these measurements on surfaces that are non-horizontal, for example inclined or even inverted. One important application for such a device would be to determine if a surface is properly prepared and ready for adhesive bonding or painting. Finally, a need exists for a device and method that can accurately determine the wetting characteristics for surfaces that may be physically or chemically heterogenous.